DNA-PK is a nuclear serine/threonine protein kinase that requires DNA for activity. DNA-PK consists of a large 470 kD catalytic polypeptide and a DNA targeting subunit known as the Ku autoantigen. Ku is a heterodimer of 70kD and 80 kD polypeptides. cDNAs that encode these polypeptides have been cloned. STEM mass measurements of the holoenzyme and the components are planned. Biochemical studies suggest that the complexes contain equal molar amounts of the subunits, but this is not very accurate. Preliminary STEM analysis of the catalytic polypeptide indicated it needed further purification.